The objectives of this study are to identify and characterize changes in subspecies of cytochrome P-450 which occur in the rat after treatment with polychlorinated biphenyls (PCBs), 2,3,7,8-tetrachloro-p-dibenzodioxin (TCDD) and other environmental chemicals and to assess the implications of these changes. Present work includes purification and characterization of a subspecies of cytochrome P-450 from livers of rats treated with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB) and comparison with P-450PB and P-450MC isolated from phenobarbital and 3-methylcholanthrene (3-MC) treated rats. The cytochrome from HCB treated rats differs from the other two in its molecular weight, peptide maps, immunological properties and catalytic activities. This cytochrome is induced by all 3-methylcholanthrene type inducers tested. An RIA has been developed for quantitation of these cytochromes in tissues.